1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. Note that in this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As an example of such a semiconductor element, for example, a transistor (a thin film transistor and the like) can be given. In addition, a semiconductor device also refers to a display device such as a liquid crystal display device.
2. Description of the Related Art
In recent years, semiconductor devices are indispensable to human life. Semiconductor elements, such as thin film transistors, included in semiconductor devices are manufactured in such a manner that a thin film such as a semiconductor film is formed over a substrate and the thin film is processed into a desired shape by a photolithography method or the like. Such a manufacturing method is used for manufacturing an active matrix liquid crystal display device (e.g., a liquid crystal television).
A semiconductor device typified by an active matrix liquid crystal display device is generally manufactured by a photolithography method. In a photolithography method, it is required to reduce the number of photomasks in view of productivity. For example, techniques by which the number of photomasks can be reduced are disclosed in Patent Documents 1 to 4.
Meanwhile, as a material for a semiconductor layer provided for a thin film transistor, silicon is widely used. On the other hand, in recent years, a thin film transistor including an oxide semiconductor as a material for a semiconductor layer has been actively researched. However, it is known that carrier concentration is greatly changed in a thin film transistor including an oxide semiconductor when moisture enters the semiconductor layer (e.g., Patent Document 5 and Patent Document 6). It is also known that characteristics of a thin film transistor including a semiconductor other than an oxide semiconductor are changed when moisture enters the semiconductor layer (e.g., Patent Document 7).